The Butterfly Effect
by SwordLover87
Summary: A story about how the tiniest change can spread outward and grow massive, like a ripple in a pond. (This is an Undertale AU.)
1. Welcoming Committee

When Frisk woke up, it was dark. Cold. Their head was buzzing. There didn't seem to be anything tethering their feet to the ground, and their limbs felt strangely heavy and unwieldy. They were sinking slowly into the blackness, their thoughts dimming and winking out one by one. But then a deep voice broke through the fuzzy feeling overtaking Frisk, a voice in their head.

 _"Human, please... wake up! This cannot be the end, when your journey has only just begun..."_

The sound snapped Frisk out of their daze. Their eyes flew open as they realized they were in a deep pool, quite a few feet underwater. Instinctively Frisk kicked out for the surface, their feet snagging on various water plants and their lungs burning. Finally, Frisk's head broke the surface, lifting several lily pads along with it. They treaded water for a few moments, taking deep, shuddering breaths, before they struck out for the edge of the pool. After clambering, dripping and gasping, onto dry land, Frisk shook themself like a dog, sending a spray of water droplets, pond scum, and small lily pads everywhere.

Frisk was about to survey the room when the voice echoed in their head again. _"Oh, it's a relief to see you are alright. I was so worried when you did not move..."_

They grabbed their head in shock. A ghost? A hallucination from lingering lack of air? As if reading their thoughts, the voice laughed gently. _"I assure you that I'm quite real. I cannot say I am much of a ghost, either..."_

Well, there was nothing else for it. They might as well get to know this person. Frisk politely extended a mental hand to the voice, asking what their name was. _"You may call me... Fluffy!"_

There was a moment's silence before Frisk doubled over laughing. There was something inexplicably hilarious about a deep, ominous voice in their head asking to be called _Fluffy_ , of all things. Fluffy seemed to agree, as after a moment, they joined in the laughter with a baritone, Santa Claus-sounding chuckle. The two shared a good laugh for a little while more before Frisk struggled upright, still wheezing. They figured they should get a move on, and glanced around before their gaze settled on the only exit.

It was a long, dark hallway. Frisk might have been scared to walk down it alone, but Fluffy's comforting presence spurred them to continue on. They steeled themself, brushed a small lily pad off their shoulder and walked forward.

The hallway proved to be innocuous, with a tall, ornate door at the end. But the room beyond was something entirely different. It was filled with stacks of scrap metal and scientific trash. Pulverized modems still spitting sparks, computers with holes seared in their screens, smashed collections of stained beakers, barrels of ominous multicolored liquids. Frisk noted that one of them appeared to be oozing ketchup as they picked their way through the trash heaps. As they walked up to a small clearing in the trash, it was no wonder that they didn't notice the mechanical flower sitting there, blank screen lolling to one side.

Until with the tiniest, the most imperceptible of movements, it straightened up.

Frisk flinched backward as Fluffy let out a ghostly gasp. With a series of clicking and whirring sounds, a simple red smiley face slowly flickered into focus on the screen. After a moment, the robotic flower spoke in a buzzing, vaguely feminine monotone. "Greetings. I am a standard service FlowerTron, here to assist you in various tasks. How shall I help you?"

Before Frisk could say a word, the FlowerTron cut in again. "No need to speak, child. You are the first human to come here in a long time, and I can see that you are in severe need of education on the rules of this world. Come! I will guide you through them."

And without warning, the world spiraled into monochrome until it was just Frisk, the robotic flower, and the comforting presence of Fluffy. A bright red heart, now the only colored thing in the general vicinity, floated out of Frisk's chest. As the FlowerTron leaned forward slightly, Fluffy offered a line of narration: _"The FlowerTron is ready to teach you."_

The FlowerTron looked off to the side slightly, its ever-smiling expression unreadable. "Child, that red heart is your SOUL. It can grow stronger by gaining LV, or LOVE, which..."

Its face flickered out for a moment.

"...I have lied to you. I am not here to help."

A chill ran down Frisk's spine. Something was wrong. They cast a surreptitious glance around the room, looking for a way out between the piles of trash but seeing none, as the FlowerTron spoke again. "I am not a standard service FlowerTron, either. They call me many things. My creator called me Aista, and others called me defective. But i think they are the defective ones."

The tone of the robotic flower's voice was changing now, sounding angry, vindictive and noticeably less mechanical. Frisk suddenly got the chilling impression that Aista was something much more than a simple broken robot as its voice got louder. "A human SOUL has such great power that they could pass through the Barrier with ONE! They have six SOULs and do not need even one more to visit their wrath on the surface... and yet they call ME defective? Ehehehehehe!"

Frisk let out a little yelp as Aista's pixelated face shifted, warped and wobbled, flicking between screaming human visages as it laughed insanely. They turned and tried to run, but two metallic, segmented tentacles burst from the floor, encircling their ankles and slamming them to the ground. Fluffy muttered a panicked _"Oh dear,"_ as Aista loomed over Frisk, its screen glowing red in the gloom.

"I will take your SOUL and cross the Barrier. I will do what those cowards failed to do! Now DIE!"

Frisk screamed as a third metal tentacle arced through the air toward their chest... but then something oblong and white flashed through the air, cutting the segmented limb aside like it was a blade of grass. More white, oblong objects- bones, Frisk realized- burst out of the ground, severing the tentacles that held Frisk's ankles. Then, to top all the insanity off, a massive draconian skull appeared out of thin air, firing a beam of energy that launched Aista through the roof of the room and out of sight.

For a few moments, there was silence as the dust settled. Then an extremely tall skeleton entered the room, picking its way through the trash piles. Its bony limbs were long and gangly, a purple turtleneck and a white labcoat draped precariously over its ribs. Its face seemed frozen in a permanently exasperated grimace. Its eyes were tiny luminous pinpricks in empty eye sockets with deep bags beneath them. It carried a clipboard under one arm, which it tapped absentmindedly as it muttered, "I MUST SEE ABOUT HAVING THAT DEFECTIVE PEST REMOVED PERMANENTLY."

The skeleton stalked up to Frisk and stood over the shuddering child. Its glowing eyelights swept their body like searchlights before it spoke again.

"MINOR INJURIES SUSTAINED. BRUISES ON ANKLES. YOU WILL LIVE. NOW GET UP."

The skeleton's voice was low and droning, with an authoritative tone. Petrified, Frisk scrabbled at the floor, trying to back away. It took out the clipboard calmly and scribbled a note before looking up, making eye contact with Frisk. "I AM DOCTOR GASTER, THE LEADING SCIENTIFIC MIND OF THE RUINS. I TEST THE HUMANS THAT FALL DOWN HERE. YOU ARE TEST SUBJECT NUMBER SIX. NOW **GET UP**. I WILL NOT HARM YOU."

Frisk struggled upright for what seemed like the thousandth time that day, and stared up at the doctor. He turned sharply and began to walk back through the piles of junk. Frisk jogged after him. Soon the scientist and the child found a door, walking through it.

It led into a room filled with water pumps, mills, and purifiers, all stationed around a beautiful river cutting the room in half. A shiny chrome bridge crossed the gap. Dr. Gaster strode across, and after a moment, Frisk followed. As Dr. Gaster strode up a stairway near the end of the room, Frisk noticed something beneath it- a pulsing yellow star. Gaster didn't seem to see it, and Frisk curiously walked up and touched it. They felt a surge of willpower and confidence that sent them reeling, and dimly heard Fluffy saying, _"However short he may be with you, Dr. Gaster saving you from Aista helps you stay determined."_

Frisk just stood there for a moment, unable to make heads or tails of what had just happened. Dr. Gaster impatiently calling them to follow him snapped them out of their daze, and they followed him up the stairs.


	2. Testing, Testing

It was bright in the Ruins. Harshly bright and smelling of antiseptic, like a doctor's office. That was what Frisk thought as they emerged into another room on the heels of the Doctor. Their boots clumped on pristine white tiles, reflecting the light from unseen lamps. Gaster strode ahead and stopped in front of a powered-down terminal set into the wall, letting Frisk take in their surroundings.

The room was small and square. A path of slightly darker tiles led from the way in to a pair of futuristic doors, gleaming chrome and tightly shut, that were placed in the far wall next to some kind of scanner. Several large, polished buttons were scattered to either side of the path. As Frisk puzzled over the odd arrangement, Gaster took out his clipboard again and begun to scribble as he spoke. "THIS IS YOUR FIRST TEST. IT IS A SIMPLE TRIAL OF YOUR SPATIAL RECOGNITION SKILLS. STEP ON ALL THE BUTTONS IN ANY ORDER YOU WOULD LIKE."

Frisk jumped on each button in rising order, not really paying attention to their movements. It seemed so easy that they didn't notice they'd stepped on the last button- until, with a click, the lights turned off, leaving them in complete darkness. Gaster's voice boomed out of the black. "PRESS THE BUTTONS AGAIN, IN THE REVERSE OF THE ORDER YOU FIRST PRESSED THEM IN."

This was significantly more of a challenge. Frisk thanked their lucky stars for muscle memory as they blindly worked their way back through the room, Fluffy offering a few helpful comments about the locations of the buttons. As soon as they once again reached the first button, the lights flipped back on. Gaster scratched down a note on his clipboard, then held his skeletal hand up to the scanner that was placed next to the doors. They opened smoothly with a hiss of hydraulics. He beckoned Frisk once before briskly walking through.

Frisk peered around the edge of the doorway and saw a narrow, lengthy room. The tiled floor was shiny, slick and wet, and after a moment Frisk saw why- deluges of water suddenly poured from the ceiling in several spots, crashing and spraying on the tiles in succession up and down the hallway. After about two seconds, the pouring water shut off abruptly.

As Frisk watched, open-mouthed, Gaster spoke from next to them. They jumped in surprise. He had somehow silently walked around them while they were looking at the water. "MAKE YOUR WAY THROUGH THE ROOM WITHOUT GETTING WET. THE WATER HAS A PATTERN AND I WOULD SUGGEST THAT YOU LEARN IT."

And with that, the doctor was gone, walking through the room so fast he was on the other side before Frisk knew what had happened- and it was then that the water started up again, and Frisk couldn't see him at all.

More deluges of water splashed on the tiles as Frisk looked at the room's setup, eyes narrowed. Fluffy's voice suddenly popped into their head. _"Maybe you should stay here and observe the water for some time. The doctor did say it had a pattern. If you learn it, perhaps you can avoid the water when it comes down!"_

Frisk stared and thought for a while, slowly puzzling it out. The pattern seemed to be in the amount of time that the water paused between deluges, but they couldn't quite figure out what it was exactly... and then they realized. The water turned on and off sequentially, creating an effect where it "traveled" up and down the room. They waited until a pause in the pattern, and then bolted for the other end of the room. The water advanced on Frisk quickly and they ended up skidding the last few feet on the slick tiles, nearly crashing into another pair of futuristic doors. But they'd made it!

They looked up, panting but exhilarated, and saw Gaster scribble down several more notes on his clipboard before stalking over to the door. He proceeded to open it by scanning his hand, in the same way he had opened the first one. "YOUR PERFORMANCE HAS BEEN SATISFACTORY SO FAR. LET US CONTINUE."

Gaster stepped through the doorway and out of sight, as Frisk scrambled to follow him.

The next room was small, rectangular, and quite strange. It contained nothing but a dented metal mannequin with a bucket for a head affixed to the ground by way of a pole, with a rusted sword dangling limply from its hand and head hanging sharply downward. Gaster pulled two objects out of his labcoat: a remote control and a moderately sized stick, sanded down to be gripped more easily. The stick he handed to Frisk; the remote he kept.

"THIS TEST IS CLASSIFIED AS COMBAT-BASED. IT IS ONE OF TWO LIKE IT. THE SECOND IS HARSHER, SO YOU WOULD BE ADVISED TO PREPARE YOURSELF NOW. STEP TOWARDS THE TRAINING DUMMY WHEN YOU ARE READY."

Frisk gripped the stick unsteadily and took a step toward the mannequin. Behind them, they could hear a beep, and they spun around in time to see Gaster press a button on the remote.

With a mechanical click and whir, the seemingly harmless, beat-up metal dummy whipped upright. The rust-flecked blade of its sword glinted with a dull light as it swished into position, and the dummy began bouncing back and forth as if daring Frisk to come closer. They gasped and stumbled back before narrowing their eyes and taking a courageous step forward.

The world spiraled in and faded to monochrome as Frisk's soul emerged. Fluffy provided a bit of narration: _"The Robo-Dummy stands its ground."_

Not wasting a moment, Frisk barreled towards the dummy, swinging their stick wildly in a chaotic hurricane of sanded cellulose. The robo-dummy countered with a flurry of surprisingly deft sword swings and parries. Not one to be deterred by this, Frisk ducked under the rusty blade before swinging their stick hard at the dummy's pole. It connected with a satisfying _clang_ and left a deep dent in the metal. The robo-dummy tilted precariously over.

Behind Frisk, Fluffy was obviously quite agitated. _"Oh, what was Gaster thinking with this test? You could get hurt!"_

Frisk, however, was pretty sure they had it under control. A series of blows to the bucket were doing wonders, as the dummy began to jerk about as if possessed. One last Little League-worthy swing did the trick, and the robo-dummy's head was smashed off its shoulders. The dummy creaked, leaned dangerously and finally fell over with a sharp clang, the exposed wires in its neck sparking and hissing like angry snakes.

The world brightened back to normal. Gaster scribbled something else on his clipboard, and left the room. Frisk elected to blow a raspberry at the metal wreckage before jogging into the next room, dragging their new stick behind them.

If that other room had been lengthy, the room Frisk was now stepping into was impossibly long. It stretched at least half a mile straight in front of them. The floor was made of mostly grey tiling, with a zigzagging black path set into the center of the room. The fluorescent lamps seemed hotter and brighter here, burning the back of Frisk's neck as they took a few more steps into the room.

Gaster loomed behind them, having snuck up silently again. "THIS TEST IS LIKELY THE MOST PHYSICALLY STRENUOUS THAT YOU WILL HAVE FACED THUS FAR. IT IS THE SECOND OF THE TWO COMBAT TESTS."

Frisk blinked- and suddenly Gaster was twenty feet away. How had he even moved that fast? His voice echoed hollowly off the walls. "THROUGHOUT THE TEST, DO NOT PANIC. THERE IS NO REAL DANGER. KEEP THIS IN MIND."

Frisk barely had the time to wonder why the doctor expected them to panic before the lights shut off.

There was a brief second of panic before the lights flicked back on, dimmer this time. Frisk's panic only grew when they realized Gaster was now nowhere to be seen. His voice echoed again, as if from far away. "MAKE IT TO THE END OF THIS ROOM."

There was a faint clanging, clattering sound. Frisk whipped around, their eyes settling on a row of evenly spaced ducts in the ceiling that they hadn't seen. The clattering sound was coming from the duct directly above them.

With absolutely no warning, a robot dropped out of the duct.

Frisk gasped and jumped backward as it hit the ground standing. Its limbs were whipping metal tubes with plastic graspers on their ends, its body a thin metal cylinder, its head a sphere with a single glowing eye rotating around it. Said eye locked onto Frisk, dilated, and narrowed, before the robot began to quickly advance on them, clicking its plastic graspers threateningly.

Frisk came in swinging, knocking the robot a heavy blow with their stick on the side of its spherical head. It listed sideways, its glowing eye swiveling madly as it struggled to regain its bearings. Frisk wasted no time battering down a rain of blows on the same spot. One final strike sent a spiderweb of cracks down the center of the globe. The robot's eye flickered, wavered, and finally blinked out. It slumped over and hit the floor with a resounding metal _crash_.

A few more distant clatters from above alerted Frisk as to their situation. Fluffy whispered urgently inside their head. _"I would suggest that you run. More of those will surely soon be coming."_

Frisk agreed and sprinted down the center of the room. Another robot fell from a duct in front of them, blocking their way. Frisk simply dodged to one side before hitting the robot in the head with a forceful swing, launching it backward. They resumed running, dodging around the occasional robot, and had nearly reached the end of the room when they chanced to look behind them.

A huge mob of robots had amassed about twenty feet away, and was advancing on Frisk slowly. Every few seconds, a new one would fall from a duct and join the crowd of automatons. Frisk picked up the pace, running faster and faster, not looking where they were going at all- until they crashed right into something hard and fell to the ground. Dazed and winded, they looked up, right into the eyesockets of Dr. Gaster, who was standing in a doorway.

The doctor was clutching a remote between his phalanges. As Frisk watched, he pressed a button and wordlessly pointed behind them. They turned just in time to see the mob of robots seize up and stop walking entirely.

With another button press, the robots were sucked back up into the ducts. The room was soon empty. The doctor looked down at Frisk as they struggled up to face him. There was a moment's silence.

"YOUR PERFORMANCE WAS... EXEMPLARY. YOU AVOIDED INJURY ENTIRELY AND INCAPACITATED A NOTABLE NUMBER OF DRONES. I AM..."

Gaster was silent for a moment.

"I AM... PROUD OF YOU. YOU DID WELL. LET US MOVE ON TO THE NEXT TEST."

With that, the doctor turned on his heel and marched into the next room.

Frisk was rather proud of themself too. As they followed Gaster, Fluffy's voice echoed bemusedly in their head. _"That was the most emotion he has shown since you first met him. I will never understand Doctor Gaster..."_


End file.
